Magnetic recording media, such as magnetic tapes, floppy disks, and hard disks, are generally produced by forming a magnetic layer, a protective layer, etc. on a non-magnetic support. Metal-deposited magnetic recording media having a ferromagnetic metal film formed by vacuum thin film formation techniques, such as sputtering and vacuum evaporation, have been put to practical use. Such metal-deposited media easily reach high magnetic energy and easily achieve a smooth surface profile by using a non-magnetic support with a smooth surface, which leads to reduced spacing loss. As a result, they exhibit excellent electromagnetic performance and are suited to high-density recording. In particular, sputtering processing is capable of achieving higher magnetic energy than vacuum evaporation processing and has been adopted in the production of recording media demanding high recording density, such as hard disks.
The manufacturers have been challenged to develop magnetic recording media capable of higher density recording, and the demand for higher electromagnetic conversion characteristics has been boosted. To meet the demand for improved recording density, it is desirable that the magnetic layer of flexible magnetic recording media having a flexible polymer film (e.g., a polyethylene terephthalate film or a polyethylene naphthalate film) as a non-magnetic support, such as magnetic tapes and floppy disks, be a thin ferromagnetic metal film formed by sputtering or vacuum evaporation.
However, thin film formation on a polymer film by sputtering or vacuum evaporation at an increased evaporation rate involves problems on account of poor heat resistance of the polymer film. For example, the non-magnetic support (i.e., a polymer film itself or a layer thereon) is thermally deformed, or the surface of the support deteriorates due to precipitation of oligomers. As a result, the surface smoothness of the support is ruined, which will lead to a failure to form a smooth magnetic layer thereon.
The following approaches have been suggested to address the problems. One is to use a heat-resistant resin as a support. Polyimide films can be expected as a heat-resistant material. This approach is impractical however because, for one thing, polyimide films are generally expensive and, for another, polyimide films having satisfactory surface properties, i.e., high smoothness, are technically difficult to make and to use.
The other approach is to provide a relatively inexpensive polymer film, which has usually been used as a flexible support in conventional particulate magnetic recording media, with an undercoating layer to improve smoothness and heat resistance of the polymer film.
For instance, JP-A-6-349042 discloses a method of fabricating a film with satisfactory surface properties by providing a resin film containing fine particles on a polymer film having a relatively rough surface. However, where an ordinary resin binder as recited in the disclosed method is used, the film undergoes serious thermal damage to its surface when a magnetic layer is formed thereon by sputtering.
JP-A-7-225934 teaches a method for suppressing thermally induced oligomer precipitation by coating a polyethylene terephthalate film with polyethylene naphthalate. Notwithstanding the use of polyethylene naphthalate, the film undergoes deterioration due to oligomer precipitation when heated to 200° C., a temperature generally adopted in sputtering.
JP-A-6-208717 proposes coating a polymer film with a more heat-resistant polyamide or polyimide resin. Application of such a heat-resistant material endows a polymer film with a heat resistance feature withstanding sputtering but involves various problems. That is, because polyamide resins and polyimide resins generally have low solubility in general-purpose solvents, they need a hard-to-handle solvent. Even where they are soluble in a general-purpose solvent, the resulting resin solutions are too viscous to afford a uniform thin coating film and meet difficulty in increasing surface properties. It is difficult to thoroughly dry the solvent, resulting in a considerable residual solvent content in the coating film, which can cause blocking between the coating film and the reverse side of the film when the film is wound. The residual solvent can also evaporate and contaminate a vacuum chamber in magnetic layer formation.
It is effective to form a film of an inorganic substance as a still more heat-resistant coat. For example, a silica coat obtained by hydrolysis of a silane compound or a metal oxide coat obtained from a metal alkoxide can be expected as a heat-resistant inorganic film. However, such an inorganic film is incapable of following the thermal expansion of a non-magnetic support and develops cracks easily. A magnetic layer formed thereon will easily develop cracks, too.
The present inventors previously proposed in JP-A-8-329443 a magnetic recording medium having an undercoating layer mainly comprising Si—O or Si—O—N, which they believe has settled the above-mentioned problems to some extent. The support used in this magnetic recording medium has a smooth surface, suffers from no deterioration nor cracks when heated in sputtering for magnetic layer formation, and does not cause blocking. A problem associated with this undercoating layer is that the undercoating composition can crawl to cause craters when applied to a polymer film support, resulting in a failure to form a uniform coat. Besides, the coating composition is incapable of repeated application to obtain a desired thickness.
To overcome this problem, the present inventors have proposed a heat-resistant undercoating layer which mainly comprises a polymer of a silane coupling agent containing an organic group having an aromatic hydrocarbon moiety. It has turned out, however, that the undercoating layer suffers knots (sesame-like projections) assumably because of non-uniform rate of undercoating layer formation. This coating defect leads to a surface defect of the magnetic layer provided thereon.